J . A m . Chem. Soc. 1991, 113, 8614-8621
8614
Figure 5. The difference in energy between the TI, Pb, bi, and Po structures as a function of electron count. Parameters used were Hii(4s) = -30.0 eV, t(4s) = 3.5: Hi,(4p) = -9.0 eV, f(4p) = 1.85. Differences in energy between a given structure and the Bi structure are plotted. See Figures 2 and 3 for figure conventions and calculational details.
One general trend is clear. In the case of two or three types of electrons (s, p, or d) the energetically lowest band exerts relatively little stereochemical control. For example, were Pettifor to have contracted the d orbitals in his calculation on the later transition elements, he would have found that C u should indeed be in the fcc structure type as is known to be experimentally correct. Similarly, we have contracted the s orbitals more than standard literature values would lead us to believe in the case of the heavier main group atoms.I3 This difference can be traced to the use of Hiickel and not extended Huckel theory. In the Huckel theory of extended solids the most bonding molecular orbital is calculated to be a t an
unrealistically low energy. As we wish to place the s band near the bottom of the p band we must therefore place the Hi, COUlombic value of the s orbitals a t a rather low energy. This choice of a low energy for the Ifii value of the s orbital in turn has an effect on the overall size of the s orbital. This is so as we use the Wolfsberg-Helmholz approximation for the off-diagonal Hjj elements. In this approximation a n increase in the magnitude of Hi, directly increases the value of Hij (see preceding section). As we have used unrealistically low Hji values to compensate for the deficiencies of Hiickel theory, we must therefore rely on small overlap integrals, S,, so that the overall magnitude of the offdiagonal elements Hij is correct. In order to have small S , values we use overly contracted Slater exponents, {, in describing our Slater-type orbitals. In our work we have used an average set of atomic parameters to resolve the trends that occur within a given row of the periodic table. One drawback of this method is that we are unable to introduce element-specific features into our calculations. However, it has been previously shown that several element-specific distortions are found in structures such as Zn, Cd, Ga, and In. As such distortions do not greatly change the ring structures of the systems it is generally not possible for our generic elemental model to resolve these distortions. For example, we were unable to explain why In has distorted from the fcc structure. Finally, it should be noted that the energetic results of a Hiickel calculation are unable to explain the stability of the P4 or As4 tetrahedron. See ref 5 for a discussion of this problem.
Acknowledgment. I would like to thank the Petroleum Research Fund, administered by the American Chemical Society, and the Rackham Graduate School for partial support of this research. The band program used in our calculations is a modified version of the program written a t Cornell University by Roald Hoffmann, M.-H. Whangbo, T. Hughbanks, S. Wijeyesekera, M. Kertesz, C. N. Wilker, and C. Zheng. I also thank Dr. G. Miller for this irreducible wedge k-space programs.
Theoretical Study on the Electronic Spectra of Model Compound I1 Complexes of Peroxidases Ping Du,**tFrank U. Axe,t Gilda H. Loew,? Silvio Canuto,*qs and Michael C. Zernert Contribution from the Molecular Research Institute, 845 Page Mill Road, Palo Alto, California 94304, and Department of Chemistry, University of Florida, Gainesville, Florida 3261 I. Received March 29, 1991 Abstract: A quantum chemical study of the electronic structure and spectra of four compound I1 iron porphyrin complexes with varying axial ligands, CI- (Hemin), pyridine, imidazole, and imidazolate (model peroxidase), has been carried out using the INDO/S method. These calculations confirm the nature of the triplet ground state as one in which the two unpaired electrons occupy the Fe-0 A* orbitals that are primarily Fe(1V) 3d,, and 3d,,. About 20%of the unpaired spin density is calculated to be on the oxygen atom, in agreement with the experimental estimate of about 25%. The calculated spectra show that charge-transfer (CT) transitions exist for all compound I1 complexes. The excitation energies for these C T transitions are estimated to be between the Q and B (Soret) bands and to have moderate oscillator strengths. Although a description of the CT excited state for each of the model systems is complex, an electron transition from the porphyrin A orbitals to the Fe-0 K* orbitals is a common component. For Hemin-11, an additional component of the CT transition wave function is an excitation from the CI 3p, orbital to the porphyrin 4e.g orbitals. For the pyridine and imidazole complexes, there is a mixing in the C T of an excitation from the porphyrin K to a ligand A* orbital. For the complex with an imidazolate ligand, mixing of porphyrin A A* excitations was found. The fact that the model compound I1 complexes with both a neutral imidazole and a deprotonated imidazolate ligand have CT transitions between Q and B does not allow us to use spectral properties to predict the actual form of this intermediate in the reaction mechanism of protein.
-
Introduction Iron-oxo compounds are proposed to participate in the biochemical cycles of several different kinds of heme enzymes to
'Molecular Research Institute.
*University of Florida. (Permanent address: Department de Fysica, Universidade de Pernambuco, Recife, Brazil.
perform essential oxidations in various biochemical pathways.'-s For instance, cytochrome c peroxidase (CCP), horseradish per( I ) Poulos, T. L. Adu. Inorg. Biochem. 1988, 7, 1. (2) Poulos, T. L.; Finzel, B. C. In Peptide and Protein Reuiews; Hearn, M . T. W.,Ed.; Marcel Dekker: New York, 1984; Vol. 4, p 115. ( 3 ) Ortiz de Montellano, P. R. Acc. Chem. Res. 1987, 20, 289.
0002-7863/9 1/ 15 13-86 14%02.50/0 0 199 1 American Chemical Society
J . Am. Chem. SOC.,Vol. 113, No. 23, 1991 8615
Model Compound II Complexes of Peroxidases oxidase (HRP), and catalase (CAT), when activated by peroxides and peracids, form an iron-oxo compound I intermediate (eq la). enz R O O H compound I + R O H (la)
+
compound I
+ AH2
compound I1 AH'
--
+ AH'
compound 11
+ AH2
+ AH'
(Ib)
+ AH'
(IC)
nonradical products
(Id)
enz
I n general, peroxidase substrate oxidation occurs in two steps involving a second, once-reduced enzyme intermediate, compound I 1 (eq Ib) which is reduced back to the original ferric resting state. The free radicals formed in eqs 1 b and I C usually dimerize to give nonradical products (eq Id). By contrast, in catalase, compound I performs simultaneous two-electron reductions of its peroxide substrate. In C C P , compound I, which is believed to be formed initially, rapidly transfers one of its oxidizing equivalents from the heme unit to a nearby residue, now thought to be Trp 191 ,6 forming a compound I1 like species, called compound ES. Compound I is also believed to be formed in the reactions of the mixed function oxygenases, the cytochrome P450s. Instead of reacting with a peroxide, P450 is activated in a more complex cycle involving two sequential electron reductions with addition , ~ oxidations of molecular oxygen in an intervening ~ t e p . ~The catalyzed by the putative iron-oxo species in P450 are different from that of CCP, H R P , and CAT. In P450, the compound I species transfers a single atom of oxygen directly to a variety of substrates. While C A T and H R P compound I complexes are stable enough to have been studied by a variety of spectroscopic t e c h n i q ~ e s , ' - the ~ putative P450-compound I has not been characterized, presumably because it has an extremely short lifetime, and the evidence for it is hence indirect. The importance of iron-oxo compounds in biological systems as well as their relevance to catalysis has stimulated the development of model heme complexes of compounds I and I1 in order to probe the chemistry of these reaction intermediates without the modulation of the protein environment.' These model compounds are found in general to mimic to some extent the catalytic behavior found at the active site of cytochrome P45Os. Similarities in chemistry and electronic absorption spectra7 of the model systems to the intact heme proteins suggest that the basic electronic structure of the heme unit in both systems is the same. However, the protein environment of the heme unit in various families of metabolizing heme proteins further modulates the differences in the manner in which they oxidize their substrates. The electronic structure of compounds I and I1 complexes of intact heme proteins, particularly H R P , as well as a number of model heme complexes has been a subject of extensive theoretical and experimental investigations. Model compounds I and 11 complexes have been treated by a number of theoretical methods, including charge iterative extended H~ckel:*~U H F INDO,Io and UHF"-13 and RHF'4v'5 a b initio calculations. In these latter studies, a model compound 1 and compound 11with pyridine ligand has been elucidated by Yamamoto et al. with a C A S S C F method.I4 These combined results all indicate that the ground state of the compound I species is a quartet porphyrin a cation radical (4) Dawson, J . H.; Elbe. K. S. Ado. Inorg. Bioinorg. Mech. 1986, 4, I . ( 5 ) Ortiz de Montellano, P. R. In Cytochrome P-450; Ortiz de Montellano, P., Ed.; Plenum Press: New York, 1986; p 217. (6) Erman, J . E.; Vitello, L. B.; Mauro, J . M.; Kraut, J. Biochemistry 1989, 28, 7992. (7) McMurry, T. J.; Groves, J . T. In Cytochrome P-450; Ortiz de Montellano. P., Ed.: Plenum Press: New York, 1986; p I . (8) Loew, G. H.; Kert, C. J.; Hjelmeland, L. M.; Kirchner, R. F. J . Am. Chem. SOC.1977, 99, 3534. (9) Hanson, L. K.; Chang, C. K.; Davis, M. S.;Fajer, J. J . Am. Chem. Soc. 1981, 103. 663. (10) Loew. G. H.; Herman, Z. S. J . Am. Chem. SOC.1980, 102, 6173. (11) Strich, A.; Veillard, A. Theor. Chim. Acto 1981, 60, 379. (12) Strich, A.; Veillard. A. N o w . J . Chim. 1983, 7 , 347. (13) Yamaguchi, K.; Takahara, Y.; Fueno, T. In Applied Quantum Chemistry; Smith, V. H., Jr., Schaefer. H. F., Morokuma, K., Eds.: Reidel: Dordrecht, 1986; p 155. (14) Yamamoto, S.;Teraoka, J.; Kashiwagi, H. J . Chem. Phys. 1988, 88, 303. (15) Yamamoto, S.;Kashiwagi, H. Chem. Phys. Lett. 1988, 145, 1 1 I .
and that compound I1 is a triplet state in which the added electron reduces the porphyrin ring, leaving the environment around the iron and the active oxygen and the nature of the Fe-O bond intact. In both compounds I and 11, two unpaired electrons are delocalized in a molecular orbital confined primarily to the Fe 3d, and 0 2p, atomic orbitals and the iron is in a formal Fe(1V) state. This observation is responsible for the very similar Fe nuclear properties for compounds I and I I . I 4 3 l 5 The experimental techniques used to characterize the ground and low-lying excited states of compounds I and I1 species include magnetic s ~ s c e p t i b i l i t y , lMossbauer ~-~~ r e s ~ n a n c e , ' ~ESR20 * ' ~ and E N D O R 2 ' spectroscopy, X-ray ~ r y s t a l l o g r a p h y , ~and ~ , ~EX~ AFS.24*25For example, quadrupole splitting from Mossbauer resonance experiment^'^^'^ on heme proteins and compounds I and 11 model complexes yields very similar values of 15 mm/s. ESR20 and E N D O R 2 ' data are all consistent with the Fe(IV) configuration of iron, and the E N D O R experiments provide a quantitative measurement of the oxygen atom spin density. Structural data on these transient species is l i ~ n i t e d . ~However, ~-~~ it is relatively well established from EXAFS24s25and X-ray ~ r y s t a l l o g r a p h ythat ~ ~ the iron-oxo bond lengths are all about 1.7
-
A. The electronic absorption spectra of compound I and compound 11 species have also been i n ~ e s t i g a t e d ' and ~ . ~exhibit ~ ~ interesting common transitions in addition to the normal heme Q and B (Soret) porphyrin a to a* transitions. Specifically, the spectra of CCP-11,28 and Hemir1-11,~~ and a model compound I1 with a pyridine ligand" show common extra absorptions in the frequency range between Q and B. The purpose of this paper is to assign the optical spectra of compound I1 complexes and particularly to account for these extra features. To this end, we have performed ROHF-SCF and C I calculations, using the semiempirical intermediate neglect of differential overlap method parameterized for spectroscopy ( I N D O / S ) on the four model compound I1 complexes. The effect of deprotonation of the imidazole ligand on the electronic absorption spectra has been studied by comparing the calculated spectra of the model complexes with imidazole and imidazolate ligands. The calculated extra features of these compound I1 complexes relative to normal heme absorptions are discussed. In addition, in these studies we have verified the description of the ground electronic state. The simpler electronic structure and lower spin multiplicity of compound I1 provide the basis for future calculations of compound I structure and spectra.
Methods All calculations of the model compound I1 complexes were carried out using a spin-restricted open-shell INDO method, which is described in detail elsewhere3'-" The electronic spectra were calculated using a (16) Theorell, J.; Ehrenberg, A. Arch. Biochem. Biophys. 1952, 41,442. (17) Chin, D.; Balch, A. L.; La Mar, G.N . J . Am. Chem. Soc. 1980,102, 1446. (18) Moss, T. G.; Ehrenberg, A.; Bearden, A. J . Biochemistry 1969, 8, 4159. (19) Harami, T.; Maeda, Y.; Morita, Y.; Trautwein, A,; Gonser, U . J . Chem. Phys. 1977, 67, 1164. (20) Schulz, C. E.; Devaney, P. W.; Winkler, H.; DeBrunner, P. G.; Doan, N.; Chiang, R.; Rutter, R.; Harger, L. P. FEBS Lett. 1979, 103, 102. (21) Roberts, R.; Hoffman, B. M.; Rutter, R.; Hager, L. P. J . Am. Chem. SOC.1981, 103, 7654. (22) Piciulo, P. L.; Ruppecht, G.;Scheidt, W. R. J . Am. Chem. Soc. 1974, 96, 5293. (23) Schapperacher, M.; Weiss, R. J . Am. Chem. SOC.1985, 107, 3736. (24) Chance, B.; Powers, L.; Ching, Y . ; Poulos, T.; Schonbaum, G. R.; Yamazaki, 1.; Paul, K. G.Arch. Biochem. Biophys. 1984, 235, 596. (25) Penner-Hahn, J . E.; Eble, K. S.; McMurry, T. J.; Renner, M.; Balch, A. L.; Groves, J . T.; Dawson, J. H.; Hodgson, K. 0.J . Am. Chem. Soc. 1986, 108, 7819. (26) Dunford, H. B. Ado. Inorg. Biochem. 1982, 4, 41. (27) Mansuy, D.; Lange, M.; Chottard, J. C. J . Am. Chem. Soc. 1979,101, 6437. (28) Yonetani, T.; Anni, H. J . Biol. Chem. 1987, 262, 9547. (29) Groves, J . T.; Watanabe, Y. J . Am. Chem. SOC.1986, 108, 507. (30) Gold, A.; Jayaraj, K.; Doppelt, P.; Weiss, R.; Chottard, G.; Bill, E.; Ding, X.; Trautwein, A. X. J . Am. Chem. SOC.1988, 110, 5756. (31) Ridley, J . E.; Zerner, M . C. Theor. Chim. Acto 1973, 32, 1 1 1 . (32) Ridley, J . E.; Zerner, M. C. Theor. Chim. Acta 1976, 43, 223.
Du et ai.
8616 J . Am. Chem. SOC.,Vol. 113, No. 23, 1991 L
t
o
Table 1. Ground-State Properties of Model Compound I1 Complexes CIDvridine imidazole imidazolate(1-) Mulliken Population
Y
0.98 2.00 0.53 1.19 1.19 1.80 1.80 1.24
#
0.96 2.00 0.55 1.22 1.22 1.77 1.77 1.23
0.99 2.00 0.55 1.20 1.20 1.79 1.79 1.17
0.95 2.00 0.52 1.21 1.21 1.77 1.78 1.27
Spin Population
x
0.77 0.77 0.214 0.21
0.78 0.78 0.20 0.20
I .45 -0.72 0.15 -0.78 0.00
1.45 -0.90 -0.7 I -0.84 -1 .oo
0.80 0.80 0.19 0.19
0.81 0.81 0.17 0.17
Net Charge Fe porphyrin ligand 0
PY
im
total
Im"
Results and Discussion I. Hemin-I1 Model Complex. C4".symmetry was imposed on
the ground state of the model Hemin-TI complex. Since there is a single electron in each CI-Fe-O T * MO, the state symmetry is )A2. Table I summarizes the Mulliken populations calculated with the R O H F - S C F wave function for the ground state. Assuming that the INDO basis is related to a valence Slater basis ~~
~
(33) Bacon, A. D. Ph.D. Dissertation, Department of Chemistry, University of Guelph, Canada, 1976. (34) Bacon, A . D.: Zerner, M. C. Theor, Chim.Acra 1979, 53, 21. ( 3 5 ) Edwards, W. D.; Zerner, M. C. Theor. Chim. Acta 1987, 72, 347. (36) Edwards, W. D.; Weiner, B.; Zerner, M. C . J . Am. Chem. Soc. 1986, 108, 2196. (37) Edwards, W. D.; Weiner. B.; Zemer, M. C. J . Phys. Chem. 1988, 92, 6180. (38) Zerner. M. C.; McKelvey, J.; Edwards, W. D. Unpublished work. See also: Pauncz, R. Spin Eigenfuncfions:Plenum Press: New York, 1979. (39) Axe, F. U.;Flowers, C.; Loew, G . H.; Waleh. A . J . Am. Chem. SOC. 1989, 1 1 1 , 7333. (40) Du, P.: Loew, G. H. J . Phys. Chem. 1991, 95, 6379. (41) Dewar, M. J . S.; Zoebisch. E. G.; Healy, E. F.; Stewart, J. J. P.J . Am. Chem. SOC.1985, 107, 3902. (42) Koenig. D. F. Acta Crystallogr.1965, 18. 663.
1.40 -0.79 0.16 -0.77 0.00
Quadrupole Splittings
Figure I . Model compound I1 structures and the coordination system. singles only CI (CI-S) within the INDO semiempirical approximations, a model which has been parametrized for spectroscopy. The configurations were generated using the Rumer diagram te~hnique.'~Oscillator strengths were also calculated for each excited state relative to the ground state. A previous application of the same method, using 15 model heme complexes, has shown its reliability in predicting the relative energies and the correct assignment of the ground spin states of ferric heme complexe~.'~I n addition, it has also been successfully used for the detailed calculation of the spectra of a number of radical heme specie^.'^.^'*" The geometry of the porphyrin macrocycle for the model complexes was based on the X-ray crystal structure of iron tetraphenylporphyrin.22 The porphyrin crystal structure was modified to porphine and idealized to D4*symmetry. Although the actual compiexes have lower symmetry, D4,+symmetry labels are used for the MOs and states calculated as they are traditionally in porphyrin chemistry. The orbitals that are primarily confined on the CI-Fe-O fragment of the molecule are labeled as u l , r I . etc. (see later discussions). The ligand T orbitals are labeled with an abbreviation of the ligand, e.g., py r3for the third T orbital of pyridine. The iron atom was placed in the porphyrin plane with the Fe-Nprphyrin distance set to 2.01 A. The Fe-0 distance in all model systems was assumed to be 1.70 A. The AMI4' optimized geometries of pyridine and imidazole were used. The Fe-pyridine distance was set at 2.03 AI4 and the Fe-imidazole distance at 2.02 A.2 The Fe-CI distance of the Hemin compound I 1 model complex was set at 2.20 A.42The orientation of the complexes was such that the pyrrole nitrogens of the porphyrin macrocycle bisect the x and y axes and the Fe binding atom of the axial ligands lies on the z axis. The pyridine and imidazole ligands are in the xz plane (Figure I ) .
1.70 -0.83 -1.01 -0.86 -1.00 1.41 0.00
1 .52b O.OOb
1.48 0.00
1.43 0.00
Anisotropic Hyperfine (KG) 273 273 57
A,,
268 267 55 ..
Experimental value 0.25, ref 21. 0.03, ref 18. Table 11. CI-Fe-0 Hemin
T
260 259 54
263 263 54
Experimental values I .46 and
MO Coefficients of Model Compound 11 of CI 3Px
Fe 3dx
0 2P, 0.90 -0.42 0.10 0.22 "21 TI. 0.32 -0.36 -0.57 'This MO and the symmetry equivalent rjy*are singly occupied,
MO
0.08 -0.67
T3X*
yielding a ground state of 3A2symmetry. Table 111. CI-Fe-0 u MO Coefficients of Model Compound I1 of Hemin MO" CI 3p, Fe 4p, Fe 3d.2 0 2p, u4
*
u,* 02 01
-0.23 -0.32 -0.65 -0.35
0.64 -0.30 0.16 -0.01
0.1 1 0.67 0.00 -0.6 1
0.26 0.50
-0.33 0.66
4 u l and u2 are doubly occupied; u3* and u4* are unoccupied.
through symmetric orthogonalization, the CI-Fe-0 T electron population on Fe (sum of 3dx, and 3d,) is about 2.4, and that on 0 (sum of 2p, and 2py) is 3.6. Although the spin density is mostly located on the Fe, there is an appreciable amount of delocalization to the oxygen atom. The sum of the spin densities on these two atoms accounts for almost all of the electron spin of the complex. Spin delocalizations to CI and the porphyrin are very small and are not listed. Table I1 shows the coefficients of the CI-Fe-0 T , MOs. The bonding and nonbonding MOs, T , , and x2,, respectively, are doubly occupied, with 7r2, localizing on CI 2p,, which can be regarded as a lone pair of Cl-. The antibonding M O T),* is mostly an out-of-phase combination of Fe 3d,, and 0 2px. It has very little contribution from the CI 2p,, consistent with the spin distributions listed in Table I. The CI-Fe-0 ryorbitals, which are not shown in Table 11, are equivalent to the T , orbitals. Table 111 shows the CI-Fe-0 u MO coefficients. The two bonding orbitals, u, and u2, are occupied, and the two antibonding orbitals, u3* and u4*, are unoccupied. Contributions of the CI 2s and 0 2s atomic orbitals to the u MOs are small and are not included in the table.
J . Am. Chem. SOC.,Vol. 113, No. 23, 1991 8617
Model Compound 11 Complexes of Peroxidases Table IV. INDO/S CI-S Calculated Excited States of Model Compound I1 of Hemin theory” state i3E 23E 3% 4% S3E 6’E 7)E 9)E 103E
experimentC x 10-3, kK mM-lcm-’
LE,
AE, kK
dominant configurations and coefficient squaredb f X IO3 polarization label 0.54, (3a2,-a2,4e,) 0.37 9.2 0 triptriplet (dX2-,,2,~3*) 0.96 10.0 0 d-d triptriplet (3a2,-u2.4e,) 0.56, ( la1,,4e,) 0.40 11.4 0 (3azU-a2.4eg)0.49. (laI,,4e,) 0.47 14.7 26 XY Q 14.3 0.06 16.7 2 XY (Ia,,,4e,) 0.53. (3a2,-a2.4e,) 0.38 0.08 17.4 8 XY CT, d-d 17.0 (K,’,d,,) 0.81 18.9 2 XY (3a2,-a2,4e,) 0.53, ( lalu,4e,) 0.42 (U,+3a2,.4eg) 0.27, (3a2,-a2,~,*)0.20, ( I a,&,) 0.09 22.2 214 XY CT 1 19.6 0.18 XY B‘ (a2+3a2,,4e,) 0.31, (laIu.4e,) 0.14, (2b2,,4e,) 0.1 I . 23.4 614 (3a2,-a2,4e,) 0.1 1 . (3a2,-a2,s3*) 0.1 1 5’A2 (3e,-r2.4e,) 0.21. (3a2,-a2,u3*) 0.16, (*2+3eg,4e,) 0.15 23.6 2 z XY B 24.4 1.26 1 13E (3a2,-a2.n3*) 0.29, (3a2,-u2,4eg) 0.22, (laIu.4e,) 0.13, 25.3 922 (a2+3a2,,4e,) 0.1 2. (2b2,,4eg) 0.04 7’A2 (3e,-a2.4e,) 0.37, (3a2,-a2,a3*) 0.12, (2eg,4e,) 0.07 26.4 1 Z 8’A2 (3a2,-a2,a3*)0.48. (n3*,4eK)0.1 1, 0.10 27.9 18 z I 03A2 (3e,-*2.4eg) 0.27, (3a2,-a2,2blU)0.1 1, (a2+3a2,.2bi,) 0.08 28.2 2 2 29.3 2 13% ( laiu.7r3*)0.73 29.8 60 14?E (a2+3a2,.4e,) 0.34. (2b2,.4eK) 0.30 163E (a2+3a2,,a3*)0.25, (a2+3a2,,4e,) 0.21, ( ~ ~ + 3 e , . a ~0.15 *) 31.7 735 XY CT2 17IE (x2+3a2,.a3*)0.26, (u2+3a2,,4e,) 0.1 5, (~,+3e,,a,*)0.14 32.2 856 XY 203E (lai,.5e,) 0.17. (al,u3*) 0.16, (~,+3e,,a,*)0.13 34.9 1074 XY 37.2 304 24)E (3eB-r2,d,,) 0.54, (~~+3e,,d,,)0.37 XY QE(SCF)= -201.817345 au. CI-S includes 27 MOs (number 51-77) of the ground-state 3A2calculated with ROHF-SCF. All low-lying states are listed. For the transitions between 16.0 and 30 kK only those with nonzero oscillator strengths are listed. Between 30 and 40 kK, only strong transitions arc listed. bMO.notations: la,, etc. are for porphyrin K MOs, d, etc. are for Fe AOs, and a’. r i ,etc. are for CI-Fe-0 MOs. See Tables 11 and I l l for definitions. A combined notation, 3a,.-a __ for example, means a mixing between the two fragment orbitals. The first is always the larger component. Reference 29. ( lal,.4e,)
The CI-S calculation for the absorption spectrum was performed with 27 active MOs, which include 9 bonding porphyrin x MOs, 6 CI-Fe-0 x and u MOs, 7 porphyrin A* MOs, 3 CI-Fe-0 x* and u* MOs, and the Fe(1V) 3dX2-,,zand 3d,. Figure 2 shows the energy diagram of these MOs, with those that are involved in the dominant configurations of the excited states being labeled. These labels show the most important mixings between some of these orbitals that reduce their symmetry from D4,, to C4”. For example, 3a2,-o2 is an out-of-phase combination of porphyrin 3azu with CI-Fe-0 u2. In this notation, the first fragment orbital always has the larger weight. The calculated transitions from the ground state, 1 3A2,to the 3E states of the model hemin-I1 are x,y polarized. The calculated excitation energies and oscillator strengths for these transitions are listed in Table 1V. The dominant configurations and the corresponding coefficient squared of each excited state are shown in the table to help analyze the major type of excitations involved in the observed transitions. For comparison, identified experimental transitions from a model Hemin-I1 studyz9 are also listed in the table. A plot of the calculated spectrum is shown in Figure 3a. All states with excitation energies below Q are listed in Table IV. These are two “triptriplet” states, resulted from the spin exchange coupling between the triplet Fe(IV) and the excited triplet porphyrin, and one Fe(1V) d - d transition. Although these transitions are calculated to have zero oscillator strengths, they are not dipole forbidden through symmetry. The normal bands of heme proteins, the Q and B (Soret), are estimated to be within 2000 cm-’ of the experimental frequencies. The Q band consists almost purely of transitions from porphyrin 3a2, and l a , , to 4e,, consistent with the original four-orbital model.43 The intense Soret Band (B), however, has a substantial amount of mixing with the charge-transfer ( C T ) excitations (CI-Fe-0 az,4eg). and (3a2,-u2,7r3*). As a result of this mixing, a “split Soret IS calculated, as it was for carbonyl ferrous P450.44 These two Soret bands are labeled as B and B’. Experimentally, however, the split Soret is not observed. There are two possible causes for the discrepancy. One is that the actual split between B and B’ is two small to be detected and the other is that the intensity of B’ is (43) Gouterman, M.; Wagniere, G.H. J . Mol. Specfrosc. 1963, 1 1 , 108. (44) Loew, G.H.; Rohmer, M . M . J . Am. Chem. SOC.1980, / 0 2 , 3655.
-.- I
-
0.0
1
- 5
-
%
-
%
n
%
v
w
-10.0
I-YY
-
I
t CI’
’
PY
im
im”
-15.0
Figure 2. MO energy diagram for model compound 11. Only active MOs in the CI-S are included. MOs up to lalu are doubly occupied. Two
electrons occupy the degenerate Fe-0 K* orbitals, short handed as n3* for hemin-I1 and K * for all other complexes. This occupancy gives the ground state of all compounds -’A,-likesymmetry (see text). substantially smaller than B and it becomes a shoulder of B (see Figure 3a). The transitions a t 17 and 22 kK a r e assigned as the extra features of the electronic spectrum observed experimentally for
Du et al.
1. Am. Chem. SOC.,Vol. 113, No. 23, 1991
8618
1.2
1.2
t 1 .o
B
CT2
(a) Hemin-ll
1.0
0.8
0.6 ~
~
0.4
CT1
(b) Compound II Pyrldlna Ligand
/I
:I 0.8
I
8'
I
B
}
!
I
n 0.2n
0.2
CTld-d 0.0
Q
I."
40
35
30
20
25
15
10
40
35
30
25
20
15
10
L
:
I 1.5 1.0
(c)
1I
(d)
Compound I1 Imidazole Ligand 1.5
.
B f
B
-
t
1
11
35
CTl
)1
1.0 .
Wsh)
O 0 .' O5 . 4 0
30
25
20
15
10
0.0 40
Compound II lmldazolate Ligand
35
30
20
15
1'
25
Figure 3. Calculated spectra for the model compound I1 complexes with the INDO method. Comparisons with experiments and detailed descriptions of the calculated excited states are given in Tables IV-VII. States with energy difference 0.2 kK or smaller are represented with a single line.
the model hemin-I1 complex. There are three weak transitions near 17 kK,two of which, 53E and 73E, are the porphyrin Soret-type ( 1a1,,4e,) and (3a2,-u2,4e,) excitations. The third, 63E, is dominated by the (r3*,d,) excitation. Since the r 3 * orbitals have coefficients on both Fe 3d, and 0 2p,, this transition is labeled with Fe(1V) d-d and CT. The absorption intensity calculated for 93E (CT2) is moderate. This state has a complex wave function characterized by two dominant configurations. Both configurations are C T in nature. The first is an excitation from u2, which is mostly CI 2p,, to 4e,, and the second involved a transition from 3a2, to a3*, which is largely on Fe(1V) d,. A third configuration contributing to this state is the porphyrin (1 alu,4eg) transition. The moderate charge-transfer absorption at 3 1.7 kK was labeled as CT2. Some z-polarized transitions of 3A2symmetry are also found for Hemin-I1 with relatively high frequencies and small oscillator strengths. Three of these states, s3A2, ?A2, and 103A2,result from a mixture of porphyrin (3e,,4eg) and C T excitations. The small oscillator strengths calculated for these transitions are not surprising as they are symmetry forbidden in D4hand gain intensity through the reduction of symmetry caused by the difference between the axial ligands 0 and CI. The fourth z-polarized transition, 83A2,is a mixture of three different C T transitions. Since all z-polarized C T transitions are weak, they are likely to be buried in the neighboring strong Soret and C T 2 bands. Single-crystal polarized absorption spectra of Hemin-11, however, may verify the presence of these predicted transitions. 11. The Pyridine Compound I1 Complex. The calculated 3A2 ground-state properties of the model compound 11 complex with pyridine as the fifth ligand (Table I) show that the electron
populations of the Fe and 0 atoms are significantly reduced, while the spin densities remain essentially unchanged from those results for Hemin-11. Fe(1V) is less positively charged because the N atom of the pyridine ligand is less electron negative than CI. The negative charge of the porphyrin moiety is only slightly reduced. Since pyridine is neutral, the total charge of the complex is zero. Because of the similar electronic environment for Fe that exists in these two complexes, similar nuclear quadruple splittings and anisotropic hyperfine constants are found. The energy diagram of the 27 active MOs used in the CI-S calculation are also shown in Figure 2. In addition to the most important porphyrin a and Fe-0 orbitals, some pyridine A and a * orbitals (denoted as py a and py a*) are included in this active orbital set. In the subsequent tables and text, the Fe-0 d,-p, and d,rp, orbitals are simply noted as A and u. Table V lists the calculated transitions for this complex. Since the pyridine complex has only C,, symmetry, the excitation energies and oscillator strengths calculated for 3B, and 3B2excited states do not match exactly. As a result, absorptions with unequal x and y polarization are predicted for the complex. This difference in the degree of x and y polarization, however, is small, indicating a minimum perturbation of the pyridine ligand to the spectra. A plot of the calculated spectrum is shown in Figure 3b. The triptriplet and d-d transitions are again found to be lowlying in energy. The Q bands are pure porphyrin excitations and the calculated frequencies of the x and y components are 15.2 and 15.4 kK. The B bands are composed of a cluster of excited states near 27 kK. These states all possess substantial C T character, with excitations from l a , , and 3a2, to A* and py r 4 * being important. There is also some mixing with the pyridine transition (py a3,py a4*). Collectively these transitions form a
J . Am. Chem. SOC.,Vol. 113, No. 23, 1991 8619
Model Compound I I Complexes of Peroxidases
Table V. INDO/S CI-S Calculated Excited States of Model Compound I1 with Pyridine Ligand theory“ state
configuration contribution* (lal,.4e,) 0.75, (3a2,,4e,) 0.17 ( I alu.4e,) 0.77, (3a2,,4e,) 0.15 ~ d* ~ .~. 2 .~0.96 r * .~ (d,2,2,7r2) 0.96 (3a2,,4e8) 0.76, la^,&,) 0.19 (3a2,,4e,) 0.79, (la1,,4e,) 0.16 (lal,.4e,) 0.54, (3a2,.4e,) 0.40 ( la,,,4eg) 0.55, (3a2,,4e,) 0.40 (a*,d,,) 0.89 ~
I
AE,
kK
9.4 9.4 10.7 10.8 12.3 12.5 15.2 15.4 18.1 18.3 19.1 19.2 20.8 21.1 22.8
experimentC c X IO-’, kK mM-lcm-l
AE, f X IO3
polarization
0 0
0 0 0 0
25 31 3 4 3
label triptriplet, triptriplet, (d-4, (d-4, triptriplet,
triptriplet, x
Q,
Y
QY (d-4, (d-4,
X
17.0
0.08
0.88 Y (3a2,.4e,) 0.67, (lal,,4e,) 0.18 Y X (3a2,.4e,) 0.74, (lal,,4e,) 0.17 1 X 6 (la,,,py r4*)0.61, (lalu.**)0.28 17.9 0.16 CTI, 37 (3a2,,7r*) 0.36, (3a2,,py *4*) 0.35 Y X (lalu.py *4*) 0.95 IO 0.14 18.4 CTI, X 169 (3a2,,**) 0.59, (2a2,,**) 0.10, (3a2,,4e,) 0.08 23.7 (3a2,,py 74.1 0.95 24.4 13 Y (PY *3?PY *4*) 0.63 24.6 z 1 24.6 IO z (3e ,4e,) 0.32, (3eg,**) 0.21 (&a*) 0.85 25.0 5 Y (**.a*) 0.85 25.1 2 X (lalu,**) 0.71 25.5 196 CT2, Y CT3, 250 (3a2,,py r4*)0.38, (lalu,**) 0.16, ( l a d e , ) 0.09 26.3 Y CT3, 31 1 X (py r3,py r4*)0.16, (lalu,**) 0.16, (la,,,**) 0.13 26.7 219 (2a2,-a.4e,) 0.34, (2b2,.4e,) 0.25, (3e,-r,2bI,) 0.1 3 26.9 Y BY (py ~ , , p yr4*)0.28, (2a2,-a,4e,) 0.19, (2b2,,4eg) 0.14 27.0 51 X 274 X (py *’.py r4*)0.32, (la,,,**) 0.11, (3a2,,4e,) 0.10 27.1 B, 444 X (la,,,py r4*)0.21, (lalu,**) 0.17, (3a2,,4e 10.14 27.4 B, 1.90 23.8 586 (3a2,,4e,) 0.20, (3a2,,py *4*) 0.18, (3a2,~r’) 0.15 27.6 Y BY z (la,,,py *s*) 0.99 28.0 2 z (3a2,,a*) 0.65 29.6 17 (3eg-7r,a*) 0.32, (nY+3e,-py r2.u*) 0.25, (3a2,,4eg) 0.09 30.8 169 Y 523 (3eg-7r.dxy)0.45, (s,+3e,,dXy)0.17, (2b2,,4e,) 0.1 1 32.7 Y 141 33.9 (2b2,,4e,) 0.16, (2a2,-a,4eg) 0.15, (a+lbl,,**) 0.10 Y X 475 (2b2,,4e,) 0.31, (2a2,-a,4eg) 0.16, ( l a d e , ) 0.07 35.1 840 35.4 (2b2,,4eg) 0.38, (3eg-*,dXy)0.13, (2a2,-a,4e,) 0.1 1 Y 37.8 714 (2a...-a.4ee) 0.57. (2b.”,,,, 4e.) 0.29 Y 931 X 38.1 (2a;,-a,4efj 0.36, ipy *,.py s4*)0.22, (2b2,,4eg) 0.14 a E(SCF) = -230.280832 au. CI-S includes 27 MOs (63-89) of the ground-state ’A2 calculated with ROHF-SCF. *See footnote b of Table IV. (**,d,,)
Reference 17. strong absorption observed as the Soret near 23.8 kK. C T transitions were found a t 21.1 and 23.7 kK ( C T I ) and at frequencies near the Soret bands (CT2). The two components of CTI are assigned as the two observed extra peaks a t 17.9 and 18.4 kK. The dominant configuration for both of these two states is C T from the porphyrin to Fe-0 (3az,,n*). Substantial mixing in the C T of transitions from the porphyrin to pyridine (3az,,py A.,*), however, is found for CTI,. As a result, CTI, is stabilized by 1.4 kK relative to CTI,. These two absorptions have moderate oscillator strengths, consistent with the experimental intensities. The C T 2 and CT3 transitions are so close to B in energies that they are unlikely to be observed. Several weak transitions between 18 and 20 kK were calculated. These are the triptriplet and Fe(1V) d - d excitations. There are also a few weak C T transitions found between 24 and 25 kK. 111. The Imidazole and Imidazolate Model Peroxidase Compound 11 Complexes. Since the Hd, atom (Figure 1) is hydrogen-bonded to an amino acid in peroxidases and can easily dissociate in the protein environment, we have included in the study of the imidazole model peroxidase compound I1 complexes the effect of deprotonation on the C T transitions. The geometries for the imidazole and imidazolate ligands were optimized with the AMI method.4’ C, symmetry was assumed for both imidazole and imidazolate complexes. The ground-state triplet is 3Aff,whose properties calculated with the ROHF-SCF wave function for both complexes show good agreement with experimental results for H R P and C C P (Table 1). The spin density on the 0 atom was calculated to be 0.20 for 0 2p, and 2p , in comparison with the experimentally inferred value of 0.25.1; The nuclear quadruple splitting of 1.52 mm/s calculated for the neutral imidazole complex
and 1.43 for the imidazolate anion complex are close to the value of 1.46 determined experimentally.18 Thus, these ground-state properties are insensitive to the deprotonation of imidazole. To explore the effect of imidazole deprotonation on the absorption spectra of model compound I1 was the next logical step of this study. Similar C I calculations were performed separately for the imidazole and the imidazolate complexes. The active MOs for CI-S shown in Figure 2 include some imidazole A orbitals, whose energies are greatly affected by deprotonation. With the building up of a negative charge on imidazole as a result of deprotonation, the imidazole A orbital energies are increased relative to the porphyrin orbitals. This relative shift of orbital energies could change the qualitative nature of C T transitions involving imidazole. The Ndlsp.z lone pair orbital of the imidazolate ligand was also included in the CI. As shown in the following discussions, excitations from this MO do not make significant contributions to the spectrum. Table VI and Figure 3c show the calculated transitions with the neutral imidazole ligand. C T transitions (CTI and CT2) with moderate oscillator strengths were found at 22.7, 23.5, 24.3, and 25.9 kK. They originate from (3aZu,r*)for C T I and (la,,,n*) for CT2. One component of each (CTI, and CT2,) is mixed with excitations from porphyrin to the imidazole r 4 * orbital. As a result of this mixing, the energies of these two states are lower than the other components (CTI, and CT2,). The two components of CT1 were calculated to have larger oscillator strengths and are assigned to the observed extra peaks for C C P a t 17.9 and 18.9 kK. The C T transitions with excitations from porphyrin A to ligand A* orbital were found to be higher in energy. Two states, 19’A” (CT3) and 2SA’ (CT4), are predominantly of transitions (lalu,im
Du
8620 J . Am. Chem. SOC.,Vol. 113. No. 23, 1991 Table VI. INDO/S CI-S Calculated Excited States of Model Compound I1 with Imidazole Ligand theory“ =>
et
al.
experimentC AE,
c X
IO’,
state 1 ’A’ 23~”
configuration contributionb kK / X IO’ polarization label kK mM-’, cm-’ triptriplet, 9.9 0 ( lal,.4e,) 0.80, (3a2,,4e,) 0.13 triptriplet, 10.1 0 (la,,.4eK)0.81, (3a2,,4e,) 0.13 10.8 0 (d-4, (d,z,z,r*) 0.96 2 3 ~ ’ 10.9 0 (dx2-,2,r*) 0.96 (d-d), 3’”’ triptriplet, 0 12.5 (3a2,.4e,) 0.80, (Ial,.4e,) 0.15 3’A’ 12.7 triptriplet, 0 (3a2,,4e,) 0.82. (Ial,,4e,) 0.14 4’”‘ 15.5 0.04 32 15.5 Q” ( lal,.4e,) 0.54. (3a2,,4e,) 0.42 4’A‘ Y 15.7 X 30 (laI,.4e,) 0.55, (3a2,&,) 0.41 5’A” 18.3 3 (r*,dxy)0.89 8’A’ I’ I8,4 2 X (r**dX,)0.89 8’”‘ 19.3 2 (3a2,.4e,) 0.76, (lal,.4e,) 0.16 9’A’ Y 19.5 1 X (3a2,,4e,) 0.80, (la1,,4e,) 0.15 9’“’ 17.9 0.13 22.7 97 (3a2,,r*) 0.56. (3a2,,im 74;) 0.14 I 4’A’ Y 23.5 83 X 13)A” ( l a l u , r * ) 0.49. (lalu,imr4*)0.27 0.12 18.9 I43 24.3 X (3a2,,r*) 0.57. (lal,,4e,) 0.08 14’A” 7 X 25.4 ( r * , u * ) 0.86 1 5’A” 25.5 3 ( r * . u * ) 0.88 I6’A’ Y z 25.7 3 (3eg-r,4e,) 0.62 1 6’A“ 144 25.9 ( l a l u . r * ) 0.84 I7’A’ Y 2 12 26.0 (3eg-r,4e,) 0.50, (3e,-r.r*) 0.12 17’A” X 26.3 1 (im r,-2b2,,im r5*)0.43, (im r,-2b2,,im r4*)0.25 18’”‘ X 78 27.0 19’A” ( I a,,.im r4*) 0.87 23.8 1.60 27.4 1178 (3a2,,4e,) 0.30. (lal,,4e,) 0.26. (3a2,.im *4*) 0.10 I9’A‘ Y X 27.6 1098 2 0 3 ~ “ (3a2,.4e,) 0.32, (lal,,4e,) 0.21, ( l a l u h nr4*)0.10 X 27.7 4 (im r,-2b2,,im r4*)0.47. (im r3-2b2,.im * 5 * ) 0.26 2 I ’A“ 28.1 4 u~ Y 2 0 3 ~ ‘ (3e,-r,4eg) 0.65, ( l a l u , 2 b l 0.14 28.2 25 2 I )A‘ (3a2,.4eJ 0.79 Y 28.4 10 ( 1 a ~ ~ . 2 b0.38. ~ , ) (2a2,-u,4eg) 0.10, (2b2,-im r3,4e,) 0. 10 23IA’ Y 1 X 28.4 (2a2,-o.4eg) 0.40, (2b2,-im r3,4e,) 0.33 22’A” z 28.7 1 (!eg-r.4e,) 0.41, (3a2,,u*) 0.12, (3eg-r,r*) 0.08 23’A” X 29.3 IO (Ial,,,im r4*)0.54, ( l a l u , r * )0.23 24’A“ X 29.4 1 (3eg-r.4e,) 0.49. (lalu,imr4*)0.12, (lal,,3b2,) 0.09 25’A” 29.6 37 (3a2,.im * 4 * ) 0.72 25’A’ Y 29.4 0.26 X 174 31.9 (3eg-r.u*) 0.45, (rX+3e,,u*) 0.38 3 1’A” 32.5 105 (3eg-r,d,,) 0.47, (r,+3e,,dX,) 0.20 32’A’ Y 33.0 31 (2a2,-a,4e,) 0.33. (2b2,-im r3,4e,) 0.32 33’A’ Y X 35.5 770 (3e,-r,d,) 0.28, (2b2,-im r3,4e,) 0.18, (2a2,-a34eg) 0 .I5 44’A” 36.0 0.72 920 35.5 (IaIu,u*) 0.16. (2b2,-im r3,4eg)0.16, (3e8-r,a*) 0.14 40’A‘ Y 35.6 186 ( l a ,,”. , , . u * l0.75 41’A’ Y X 36.4 312 (3eg-r,d,,) 0.49, (rY+3e,-im r2.dxy)0.23 47)A” 36.5 183 (3eg-r,a*) 0.38, (rY+3e,-im r p * ) 0.23, (2b2,-im r3,4e,) 0.12 44’A’ Y 38.2 503 (2a2,-u.4e,) 0.42, (2b2,-im r3,4e,) 0.22 49’A’ Y X 38.4 226 (2a2,-u,4e,) 0.32, (2b2,-im r3,4e,) 0.20, (3a2,,u*) 0.19 54’A” X 38.6 101 ( laI,,3b2,) 0.17. (rY+3e,-im r2,4e,) 0.16, (2a2,-a,4eg) 0.1 1 45”” 38.6 V 75 50’A‘ (, r . 4 ~ 0.46. ) (im r,.4e.) 0.1 I . (2a9..-u.4e.l 0.09 -” “E(SCF) = -227.31 5 386 au. CI-S includes 23 MOs (63-85) of the ground-state ’A“ calculated with ROHF-SCF. bSee footnote b of Table IV. Reference 28. >
I
8’
a4*)and (3a2,,im
~
7r4*).
_I
~
6,
Their energies are estimated to be 27.0
and 29.6 k K , respectively. CT3 is very close to the calculated Soret band and would be buried under it. C T 4 has a small oscillator strength and is near the shoulder of the Soret experimentally observed a t 24.9 kK. There are a series of strong multiconfigurational transitions near 35-36 kK,and they are assigned to the observed absorptions a t 36 kK. The calculated spectrum of the imidazolate ligand complex is shown in Table VI1 and Figure 3d. Similar to the results found for the neutral imidazole complex, CTI was calculated to be near 24 kK with a dominant configuration (3a2,,r*). The oscillator strength, however, is substantially larger than that of the imidazole complex. Another contrast to the neutral ligand is that there is no mixing in porphyrin to Fe(1V) CT transitions of excitation that involve the ligand. As a result, the CT, and CT, components are nearly degenerate. The excitation energy of CT2 ( l a , , , r * ) was found to be higher than B. CT transitions from im 7rj to 4e, ( 103A”, I 63A’, and 1 23A”, noted as CT3) were found to be rather low in energy, presumably because of the high MO energy of im 7r3 (Figure 2). The oscillator strengths for these transitions are however very small. The clustered states between 26 and 27 k K with significant intensities are assigned to the Soret bands (B) because they all are due to porphyrin (r,7r*)excitations. Since the frequencies
of these transitions are very close together, they would appear as a single peak. The strong transition around 35 kK was assigned to an observed absorption a t 36.0 kK. In all of the excited states calculated, no significant contribution of the excitations from the N,, lone pair electrons were found. These results show that the model compound I1 complex with both the neutral and deprotonated anion form of imidazole as a ligand display extra CT features. The absorption frequencies of these CT, CTI specifically, are between the Q and the B. The dominant configurations of these states for both complexes a r e the electron excitation from a porphyrin orbital to the Fe-0 7r* orbital. The major difference is that, in the neutral complex, there is substantial mixing of porphyrin to ligand CT configuration (la,,,im 7r4*) in the wave function. This mixing is a direct result of the relatively low orbital energy for the r4*of neutral imidazole (Figure 2). Consequently the degenerate CT transition in the neutral ligand complex is split, with the energy of CT, being lowered. Conclusion Charge-transfer transitions that account for the extra observed features between the Q and B bands were found for all model compound 11 complexes studied. These CT transitions have calculated oscillator strengths intermediate between those cal-
J . Am. Chem. Soc., Vol. 113, No. 23, 1991
Model Compound I I Complexes of Peroxidases
Table VII. INDO/S CI-S Calculated Excited States of Model Compound I 1 with Imidazolate Ligand theory‘ AE, state configuration contributionb kK f X IO’ polarization 0 9.7 I’A’ (la,,,.4ee)0.68. (3a7,,,4e.) 0.24 0 9.9 2’A” ( 1 a;;,4e:) 0.68, (3a;;,4e:) 0.25 10.4 10.6 11.8
2’A‘
3’A” 3’A‘
12.0
4’“‘ 4’A‘ 5IA” 7’”’
I6’A’
15.1 15.3 17.7 17.9 21.5 21.7
I2
22.0
8’A‘ IO’A” 3 ~ “
0 0 0 0
label triptriplet,
triptriplet, (d-d), (d-d),
triptriplet, triptriplet,
20 20 2 3 3 1 3
Y
557
Y
X X
Y X
Y X
15.5
0.04
17.9
0.13
BY B,
23.8
1.60
CT2, CT2,
29.4
0.26
36.0
0.72
Q, QX
(d-4, (d-4,
CT3, CT3, CT3, CTI, CTI,
14’A”
462
15’”’
25.0
81
Z
19’A‘ 17’”’
24.8 25.9
9 1
Y
203~‘ 2 I ’A’
25.4 26.1
Y Y
(d-4, (d-d), B(sh),
18’A” 223~’
26.3 27.3
x
BW),
24’A‘
27.4 27.6
6 234 23 1 8 523 566 2
I9’A” 20”” 22””
27.9 28.7 28.9 29.1
28’A’ 25’A” 44’A’ 43”” 46’A’
34.8 35.2 35.2 35.7 35.8
49’A’ 46’A”
2
experimentC x 103, kK mM-’cm-l
AE,
24.7 24.7
18IA‘
8621
X
X
Y
Y X
xz xz
54 46 22 1 404
Y
233 1116 656
Y Y
X
Y X
X
38.0 54’A’ 34 1 Y 38.0 219 X 53’A” . . . . ‘E(SCF) = -226.157 155 au. CI-S includes 25 MOs (61-85) of the ground-state ’A” calculated with ROHF-SCF. *See footnote 6 of Table IV. Reference 28. e
-
culated for Q and B. The type of charge transfer is however somewhat different in each complex. In hemin-11, both the oxygen and chloride ligands are involved in C T to the porphyrin. In the pyridine, imidazole, and imidazolate compound 11 complexes, the charge transfer is predominantly from the porphyrin a to the F e O A* orbital, but with a different extent of mixing of C T from porphyrin to ligand, which occurs significantly in the pyridine and neutral imidazole complexes. The existence of these C T transition states in relatively low excitation energy regions indicates a possible participation of these transitions in the mechanism of the compound 11 reduction to the enzyme resting state and in the mechanism of electron transfer from cytochrome c to CCP. Comparison of the calculated electronic spectra for the protonated and deprotonated species of the imidazole complex with experimental spectra of the protein-compound 11, however, did not allow a definitive conclusion to be drawn about the extent of deprotonation of the imidazole. While, as noted, the deprotonation changes the involvement of the ligand in the C T , the observed extra bands are present in both calculations, with only minor variation in intensity and wavelength. The calculated spectra of the model compound I1 complexes match the experiments reasonably well in a qualitative sense. Quantitatively, the experimental excitation energies for the CT transitions are generally lower than the theoretical estimates. The overestimate of excitation energies may be associated with an
assumed porphine geometry, particularly the Fe-O distance that remains unchanged in the CT transition, an experiment referring to a system in solution, or a limited configuration interaction. Some general trends are found for the effect of the overall absorption spectra by changing the ligand. First, the Q band is pure porphyrin (n,a*) excitations and its frequency and oscillator strength remain constant. Second, there are large changes in the Soret bands because they mix with CT transitions that involve the ligand orbitals. It is predicted that Hemin-I1 has a split Soret. The Soret bands of the imidazolate complex also consist of two sets of distinct excited states though much closer to each other in energy. These findings are consistent with the early studies on P450, in which the split Soret was predicted as a general phenomenon for an Fe-porphyrin complex with an axial ligand that has a lone electron pair. Third, the CT transition from porphyrin to Fe-0 a* is common for all model complexes studied and they lie between the Q and B in excitation energies. There is mixing of this CT state with the ligand excitations. If the ligand is planar and has a low-lying a* orbital, e.g., pyridine or imidazole, this mixing results in a small splitting of the degeneracy.
Acknowledgment. This work was supported in part through N S F Grant DMB-9096181 (P.D. and G.H.L.) and through a grant from Eastern Kodak Company (M.C.Z.). S.C. is grateful for support from C N P g for his stay a t the University of Florida.
